Proposals for electrostatic coating of tablets have been made for at least the last thirty years or so. For example, GB-1075404 (published in 1967) proposes an apparatus for coating tablets in which a liquid is sprayed onto one face of each tablet core as the tablet cores are conveyed below a first stage sprayer having an associated high voltage grid, the coating is dried, the coated cores are then conveyed below a second stage sprayer having an associated high voltage grid with the other side of the tablets uppermost, and then that coating is dried again.
Various paper proposals for electrostatically coating tablet cores with a liquid or a dry powder have been made but as yet at least in the case of pharmaceutical tablets there is no recognized electrostatic coating method or apparatus that has proved sufficiently successful to be applied commercially on a reasonable scale. While there are rotary tablet presses capable of producing pharmaceutical tablet cores continuously at a rate of for example 5,000 tablets per minute, the subsequent coating of the tablet cores is most commonly carried out as a batch process by applying a liquid coating in a revolving drum.
In order to provide a commercially viable apparatus or method for coating medicinal products various problems must be overcome. It is in many ways easier to apply a liquid rather than a dry powder as the coating material and therefore, although both options have been considered in research, workers have flavored the use of liquids: If a dry powder is applied then it is harder to obtain adhesion of the coating to the substrate, which is not itself likely to be sufficiently electrically conducting, even when the powder is electrostatically charged. In order to provide a lasting bond between the substrate and the powder, the powder must be transferred into a film, for example by melting, but in the case of a medicinal product, which in many cases will include organic materials, must not be damaged. Furthermore an even coating is required and it is very difficult to obtain an even coating of powder on an electrically insulating medicinal substrate, even when the powder is electrostatically charged.
When liquid coating is used, the coating must be dried. Theoretically such drying could in some circumstances be carried out at room temperature but in commercial practice it is important, for example because of the rate at which the process must be carried out, to heat the tablets and that is expensive because of the large input of energy required to vapourize the solvent used in the liquid coating. Another disadvantage of liquid coating is that it cannot be used for coating materials that are not soluble or suitably dispersible in a usable liquid, preferably water.